The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Three-way and selective catalytic reduction (SCR) catalysts reduce emissions in exhaust from an engine. When the air/fuel ratio of the engine is rich or stoichiometric, the three-way catalyst reduces hydrocarbon, carbon monoxide, and nitrogen oxide and produces ammonia, and the SCR catalyst stores the ammonia. When the air/fuel ratio is lean, the three-way catalyst reduces hydrocarbon and carbon monoxide, and the ammonia stored in the SCR catalyst is used to reduce nitrogen oxide. Thus, the amount of ammonia stored in the SCR catalyst decreases when the air/fuel ratio is lean.
Typically, the air/fuel ratio is adjusted to lean to improve fuel economy. Passive SCR systems may switch the air/fuel ratio from lean to rich to increase ammonia storage levels in the SCR catalyst. Active SCR systems inject a dosing agent, such as urea, into exhaust to increase ammonia storage levels in the SCR catalyst. The dosing agent breaks down to form ammonia that is stored in the SCR catalyst.